1. Field of the Invention
The present invention generally relates to the field of data encoding and scanning systems, and more specifically to an optical scanning system using a liquid crystal modulator to encode an object identification code and related sensor data.
2. Description of the Related Art
Data acquisition systems commonly use printed bar codes to identify objects for the purpose of keeping inventory, tracking manufacturing processes, and acquiring data. For example, oil refineries identify thousands of valves by unique bar codes and gather sensor data for each valve, and grocery stores use bar codes to identify prices and continually update inventories. A wand reader for scanning bar codes to identify products is disclosed in Marshall, "The Computer Comes to the Supermarket", OPTICS NEWS, January 1976, pp. 5-9.
A bar code is a series of black and white parallel bars of varying widths imprinted on a piece of paper, and formatted to represent a binary code that can be detected optically by reflecting light off the bar code and detecting the intensity pattern of the reflected light. There are many different bar code formats of which the UPC code found in grocery stores is the most widely known. A bar code includes several alphanumeric characters with each character being represented by a fixed number of black and white bars and having a predetermined total width. To prevent and identify scanning errors, the bar code format incorporates error correction codes, and includes known patterns at both ends to identify the beginning and ending of a bar code, and between the individual characters. Additionally, each alphanumeric character is printed on the bar code in case the wand won't read the code, and then it has to be entered manually.
One specific example of a bar code is code 39 that includes a total of 9 bars for each character of which 3 are wide bars and 6 are narrow bars, and an intercharacter space for separating adjacent characters. In the following examples, the wide bars have a width of 3 units which corresponds to 3 bits, the narrow bars have a width of 1 unit, and the space has a width of 2 units such that each character is represented by a total of 17 bits. The format also specifies that the first and fifteenth bits always correspond to black bars, and the last two bits (the space) correspond to white spaces. The identification codes are three characters long, and each character has 36 possibilities; letters A-Z and numbers 0-9, which provides 36.sup.3 =46,565 possible ID codes.
FIG. 1 is a diagram of a data acquisition system using a wand reader to scan bar codes and a portable sensor to gather data. In this particular example, several thousand valves are dispersed throughout an oil refinery and imprinted with bar codes to identify the valve. A technician scans the bar code for each valve to record its identification code, and uses a "sniffer" to sense and record the level of volatile organic compounds (VOC) leaking out of the valve.
A data acquisition unit 10 includes a wand reader 12 and a sniffer 14 electrically connected to a portable computer 16. A bar code 18 is affixed to a valve 20 which controls the flow of oil through pipes 21, and includes a sequence of parallel black and white bars 22 and 24 formatted in accordance with code 39 to identify the valve. The black and white portions of the code alternately absorb and reflect light. In reality, the bar codes are not pure black and white but rather grey tones, which reduces the reflected light's contrast ratio to approximately 3:1.
The wand reader includes a light emitting diode (LED) source 26 and a phototransistor detector 28. The bar code is detected by placing the wand on one end of the bar code, and scanning across the code. The LED emits a beam of light 30 through an opening 32 in the wand, and the bar code modulatse the beam between binary intensity levels, and reflects it back through the opening to the detector which converts the low-power light into a relatively high-level electronic signal that varies in proportion to the light's intensity. The electronic signal is transmitted to the computer and is routed through logic circuitry to identify the codeword.
The computer checks the codeword's validity, and if an error occurs, the code is rescanned or entered manually. Starting the wand in the wrong place, not maintaining good contact between the wand and the code, not finishing the code, etc. may cause detection errors. Furthermore, the software used to decode the modulated light must compensate for variations in scanning rate and the limited contrast ratio of the modulated signal.
After the bar code is correctly detected, the technician uses the sniffer which includes a VOC sensor 33 to sense and record the level of hydrocarbons emitted from the valve. The sensed data is digitized and stored in the portable computer.
For applications such as the oil refinery, a data acquisition system that requires technicians to use a wand reader to scan bar codes and a separate sensor to gather data is too restrictive. It is a labor and time intensive process prone to detection errors. Systems that attempt to detect bar codes automatically from a distance have other deficiencies. It is difficult to focus the light source, typically a laser beam, to a small enough point on the bar code to ensure that the light is encoded correctly. If the beam is too wide, it will simultaneously read more than one bit of the bar code. Additionally, as the length of the optical path increases, it becomes more difficult to control the path to ensure that the light is reflected back to the detector. Small variations can cause the light to completely miss the detector. Furthermore, the light reflected from the bar code is scattered. Thus as the detector moves farther away from the bar code, the perceived contrast ratio between the intensity levels diminishes to a point at which detection is very difficult. Constantly adjusting the laser beam's intensity to maintain adequate contrast is not feasible because of the power requirements and the safety hazards. If a constant beam intensity is used, very large and expensive optical lenses are required to gather enough reflected light to maintain a sufficient contrast level for detection. Another drawback is that these systems are limited to the specific task of reading bar codes. Applications that output data obtained from sensing characteristics of the object to which the bar code is attached, in addition to reading the bar code itself, require a separate system for the sensing function.
An alternative to bar codes for identifying objects is discussed in Schneiderman, "RFID Tags Locate Growing Wireless Market", Microwaves & RF, February 1994, pp. 31-36. A radio frequency (RF) transponder is programmed with the desired identification information and attached to an object. When a source illuminates the object with an RF signal, the transponder transmits an RF signal with the stored information. This system does not encode and transmit sensory data about the object, and the transmission of RF signals poses questions concerning health and safety.
The Wireless Remote Link System disclosed in the 1992 "Condensed Product Catalog" for International Sensor Technology monitors toxic and combustible gases. The principal means of communication, as well as power, is the AC power lines which are connected to a central computer. This approach avoids external scanners or RF transmission, but requires AC power lines for each unit.